Summary of Work: We use the powerful bacteriophage T4 model system to study mechanisms of mutagenesis and DNA repair. We have developed a new T4 mutation reporter gene, the rI gene, which is useful for mutation studies because it is not involved in DNA metabolism, is small enough to be accommodated by a single DNA sequencing lane, and is sensitive to missense mutations. The rate of spontaneous mutation is normal in this gene. A dramatic mutational hot-spot is observed, consisting of three adjacent base pair substitutions that arise simultaneously; this mutation is associated with an imperfect palindrome of considerable length, and the mutation occurs at the highest rate ever observed for a palindrome-driven complex mutation. In addition, we also observed a mutation consisting of six base pair substitutions scattered over 17 base pairs; in this case, the associated imperfect palindrome strongly overlaps itself, providing information useful for modelling the formation of complex mutations.